Titanium-base alloys



United States Patent 3,340,051 TITANIUM-BASE ALLOYS Evan William Evans, Hagley, and Michael Duncan Smith,

Shenstoue, England, assignors to Imperial Metal Industries (Kynoch) Limited, London, England, a eorpora- This invention relates to titanium-base alloys having a high Youngs modulus and to a method of heat-treatment for such alloys.

In order to be competitive with steels on the grounds of weight and stiffness, titanium-base alloys require a Youngs modulus greater than 20x10 lbs/sq. in. One

alloy which meets this requirement has the nominal composition Ti-A11.5-B, but is difficult to process and cracking frequently occurs during hot working. The especial care needed during processing to avoid cracking and the close inspection of the processed alloy required to detect cracked material adds to the cost of production.

The present invention is directed to modified compositions which have greater resistance to cracking during hot working whilst retaining good Youngs modulus values and adequate ductility.

According to the invention, a titanium-base alloy consists of up to by Weight of chromium and O.52% of boron, balance, apart from usual impurities, titanium.

Also according to the invention, a titanium-base alloy consists of up to 10% by Weight of chromium, 37% of aluminium, '0.5'-2% of boron, balance, apart from usual impurities, titanium, but preferably the alloy contains 47% aluminium.

The usual impurity elements include carbon, nitrogen and oxygen, which may be present at thenormal commercial levels.

The preferred range of composition for the element chromium is 210%.

Alloys in combination of high Youngs modulus, ultimate tensile strength and ductility, and are easier to process than the Ti5-All.5-B alloy since the ductility in hot working is improved. Forgeability is improved, forged stock being free from surface and internal cracks. In the heat-treated condition the alloy is useful in rod and sheet form for structural applications.

Increasing the chromium content increases the modulus but reduces the ductility somewhat and a chromium content of about 5% with a boron content of about 1.2% has been found to give a good combination of Youngs modulus and ductility. A further improvement in tensile strength and modulus is obtained when aluminium is added, there being what appears to be a synergistic eflFect.

The chromium content is therefore reduced from that accordance with the invention possess a good 3,340,051 Patented Sept. 5, 1967 ice mentioned above when aluminium is present and a Youngs modulus of 21X 10 lbs./ sq. in. is obtained with 3% chromium, 6% aluminium and 1% boron. Chromium contents as high as. 7% produce a further increase in modulus, but ductility is adversely afiected. Increasing aluminium and boron content also increases the modulus.

Quaternary alloys tend to give higher values of Youngs modulus than ternary alloys, although in both types Of alloy, modulus, tensile strength and ductility may be controlled by varying heat-treatment conditions.

In order to develop this desirable combination of properties, the alloys are subjected to a heat-treatment comprising solution-treating at a temperature in the alpha plus beta or the beta field, quenching, and ageing at as high a temperature as possible below the eute-ctoid temperature.

Solution-treatment may be carried out at about 750 C. in the alpha plus beta field and ageing may be carried out at about 650 C. The-period of ageing is important to produce maximum modulus values which, in general, oc cur in the period 20 to 160 hours, dependent on composition.

Composition atfects the duration of ageing treatment and alloys having greater chromium contents age to high modulus more rapidly than the lower chromium contents.

The effects of composition and of various ageing times on Youngs modulus are shown in Table I and on the tensile properties in Table II. It will be seen from these tables that certain alloys require a long ageing treatment and others need a shorter treatment, in order to develop a good combination of strength and ductility. The improvement in modulus brought about by the addition of aluminium is clearly demonstrated in Table I.

In general, the ageing of the alloys initially produced an increase in modulus, though rate varied with the composition, and after a maximum modulus was reached, the alloys slowly over-aged. Alloys with high chromium and aluminium content had a considerably higher rate of ageing, maximum modulus being approached after about 18 hours ageing.

The results of tensile tests are shown in Table II. Tensile strength of the alloys of the Ti-Cr-B type is high in the water-quenched condition and falls on ageing. In-

crease of chromium and boron content increases strength.

Prolonged ageing reduces the ductility of these alloys.

Structures of the alloys depend upon the degree of 1 beta stabilisation. The ternary and quaternary alloys with low chromium contents have in the quenched condition equiaxed alpha-beta structures containing boride particles, and structure is not appreciably changed by ageing. Ternary alloys of high chromium content, as quenched, have a structure of boride particles dispersed in a retained beta matrix whilst in the quarternary containing aluminium, the matrix consists of martensitically transformed beta. On ageing, retained beta and martensite alike break down to give fine duplex structures.

TABLE I.COMPOSITION AND ELASTIC MolitilflIogt TITANIUM-CHROMIUM-ALUMINIUM-BORON Nominal composition, Analysed compo Modulus of elasticity (lbs. lsq. in. X 10 WQ from 750 C. and aged at 651 0.

wt. percent sition, Wt. percent for: (in hours) Cr Al B Cr Al B 0 1 4 10 17 18 61% 1, 18

TABLE II.TENSILE PROP ERTIES OF TITANIUM-CHROMIUM-ALU- ALLOYS, WATER-QUENCHED FROM 75 C. AND

MINIUM-BORON AGED AT 60 C.

Brittle fracture in, or near test piece head.

References Cited We claim:

1. A titanium-base alloy consisting of 2-10% by weight of chromium and (LS-2% of boron, balance, apart from usual impurities, titanium.

5 .5 5555 UHUU it 7777 j:

s I: m II E M .1 Pnmnm mua e k M weww T m m m r IJJB 2565 E5556 T9999 1111 5 78 9440 8508 A 6649 9259 2223 2. A titanium-base alloy according to claim 1 consistingof 5% chromium and 1.2% boron, balance, apart from usual impurities, titanium.

s w a a N9 cTn N C O Rmm Q M r k4 05 mm mm H m 3. A titanium-base alloy consisting of, by weight, 5% chromium and 0.8% boron, balance titanium with impurities.

Titanium Project, The Manufacture of Titanium Al- 4. A titanium-base alloy consisting of, by weight, 5% chromium and 1% boron, balance titanium with impurities.

5. A titanium-base alloy consisting of, by weight, 7% chromium and 1% boron, balance titanium with impuri- 50 ties.

6. A titanium-base alloy consisting of, by weight, 9% chromium and 1% boron, balance titanium with impuri- DAVID RECK Pnmary Emmmer' ties.

C. N. LOVELL, Assistant Examiner. 

1. A TITANIUM-BASE ALLOY CONSISTING OF 2-10% BY WEIGHT OF CHROMIUM AND 0.5-2% OF BORON, BALANCE, APART FROM USUAL IMPURITIES, TITANIUM. 